The invention relates to compositions, methods, and apparatuses for improving the recovery of valuable aluminum from the aluminum hydroxide production process of a Bayer process. In particular, the invention relates to the compositions and methods providing the increase of particle size of aluminum hydroxide product without a significant decrease in precipitation yield.
As described for example in U.S. Pat. Nos. 7,771,681, 7,976,820, 7,976,821, and US Published Patent Application 2007/0172405, aluminum hydroxide is produced on an industrial scale by well-established methods such as the Bayer process. The Bayer process comprises a digestion stage in which alumina is extracted from bauxite ore, followed by a clarification stage in which red mud is removed, followed by a precipitation process in which aluminum hydroxide crystals are grown followed by a classification stage in which crystal seeds are separated from product material and followed by a calcination stage in which the aluminum end product is produced.
As described in European Patent Specification EP 0465055B1, in the precipitation process, seed material is added to liquor pregnant with alumina. The seeds function as surface sites which aid in the nucleation and growth of alumina bearing crystals of a particular distribution of specific sizes. In particular some crystals are targeted to having a size optimized to facilitate easy and efficient separation from the liquor and further processing. Other crystals are targeted to having a size optimized for functioning as future seeds.
The precipitation process operators optimize their methods so as to produce the greatest possible yield from the aluminate process liquors while trying to achieve a given crystal size distribution of aluminum hydroxide product. It is desirable in most instances to obtain the product of relatively large crystal size since this is beneficial in subsequent processing steps required to produce aluminum metal. Production is often limited by processing conditions under which the crystallization and precipitation is conducted. These processing conditions vary from one plant to the next and include, but are not limited to, temperature profiles, seed charge, seed crystal surface area, purge of carbon dioxide or flue gases, liquor loading, liquor purity, and the like.
The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “prior art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 CFR §1.56(a) exists.